The present invention relates generally to switching-mode dc/dc power converters, and more particularly to such power converters of the transformer-coupled flyback type which supply a train of current pulses to charge an energy-storage capacitor coupled to the transformer secondary winding via a diode. Each pulse in the train supplies part of the total charge delivered to the capacitor. Typical applications are to charge an energy-storage capacitor which supplies voltage to a load which subsequently discharges the capacitor. The power converter may thereafter recharge the capacitor if the load is a repetitive-discharging load. Examples of loads powered by an energy-storage capacitor are a xenon-filled flash lamp used as a photographic flash lamp, a stroboscopic light source, or a flashing light on an automotive vehicle such as a police vehicle or an emergency-service truck; an ignition/discharge system, such as a gas-ignition system; a cigarette lighter; a pulse-forming network for use in a pulsed radar transmitter; or a fuse in a military artillery shell.
A number of references preceded by bracketed numbers are listed below. These bracketed numbers appear in the description below to identify a correspondingly numbered reference pertinent to a portion in the description preceding the bracketed number.
The prior art includes hysteretic current-mode control. The transistor is turned "off" when the primary-winding current rises to the peak value given in equations (19) or (19a) below, and is turned "on" again when the transformer secondary-winding current falls to the fraction a of its initial value. This is a form of hysteretic current-mode control [4], [5]. The currents can be sensed using current-sensing resistors and/or current transformers.
U.S. Pat. No. 4,070,699 (issued Jan. 24, 1978 to Herbert M. Einbinder and assigned to Datascope Corp., Paramus, N.J.) describes a circuit which implements this control method by using current-sensing resistors in series with the transformer primary and secondary windings to sense the currents in those windings. The signals from the two current-sensing resistors are combined with relative weights corresponding to the transformer primary-to-secondary turns ratio. That combined signal is input to a trigger circuit which has two trigger values corresponding to maximum and minimum values of magnetic field strength for turning the switch "off" and "on" respectively.
U.S. Pat. No. 4,489,369 (issued Dec. 18, 1984, to Howard S. Ginsberg and assigned to the United States of America) discloses a method in which the secondary current falls to zero before the switch is turned "on" again; i.e., a is chosen as zero. This is done to ensure that all of the energy stored in the transformer is delivered to the capacitor, on each pulse. However, it nearly doubles the required peak current, increasing it by a factor of (1+a) as compared to the optimum method described here. The rms currents increase by slightly less than a factor of .sqroot. (4/3), and the i.sup.2 R power losses (in the transistor, the transformer windings, the diode, the ESR of the energy-storage capacitor, and the resistance of the D.C. supply and/or its V.sub.CC bypass capacitor) increase by a factor of almost 4/3=1.33.
U.S. Pat. No. 4,104,714 (issued Aug. 1, 1978, to Richard Hanley Smith and Peter Graham Laws, and assigned to Plessey Handel und Investments AG, of Zug, Switzerland) also requires the secondary current to fall to zero before the switch is turned "on" again, with the same increase of peak current and i.sup.2 R power losses. One way they show to sense the termination of the secondary current is to sense the reversal of the diode voltage.